Add check for database connection

[RamiAwar]

🚀 Feature Request

Raise error if database connection is not configured

🔈 Motivation

Currently, mongomantic does not raise any errors if a mongodb connection is not established. This should be done as soon as any db calls are attempted.

🛰 Alternatives

Provide default arguments? Nah.

[github-actions]

Hello @RamiAwar, thank you for your interest in our work!

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